The concept of reflex has played a major role in the attempt to understand the neurophysiological bases of the control and coordination of movements. Reflexes have been attractive theoretically, because through these pathways, sensory feedback can alter activity in muscles and thus contribute to the stability of the organism and to the production of the optimal pattern of movement. Current knowledge of reflex control of movement is in large part based on experiments focusing on spinal circuitry in experimental animals. Knowledge of reflex pathways in the human oral motor system is limited, and there is a need to design and carry out experiments that describe basic properties of human oral motor reflex pathways and the role played by these pathways in behaviors such as speech and mastication. Such knowledge is essential as a basis for understanding a variety of disorders involving the craniofacial system. Previous research has demonstrated that under well-controlled isometric conditions, activity of the human jaw muscles is extremely sensitive to modulation by sensory feedback arising from low-threshold intraoral and perioral cutaneous mechanoreceptors, from jaw muscle afferents, and from auditory pathways. These experiments have provided clear evidence of the existence of a set of potentially powerful synaptic inputs to human jaw muscles. The proposed research is designed to answer these questions concerning basic physiological properties of oral reflexes operating on human jaw-closing and opening muscles: (1) To what degree does individual variation in the amplitude of reflex responses develop through use and to what degree are these differences inherited? (2) What are the effects of the developmental and aging processes on the nature of human reflex responses? (3) Are the effects of oral motor reflex pathways on motoneurons normally suppressed during oral motor behaviors, or do inputs generated over reflex pathways contribute to the control and guidance of complex movements? (4) Can abnormal transmission characteristics in oral motor reflex pathways lead to stuttering or other disorders of movement? These questions will be addressed by applying innocuous tactile, stretch, and auditory stimuli while human subjects either maintain specified static background conditions or while they speak. Responses to these stimuli will be measured as changes in electromyograms (EMG's) recorded from jaw-closing and opening muscles, in jaw position, and in mandibular force output.